An increasing number of electrical and electronic systems are being designed to be powered by batteries. Prime examples of such systems include portable computers such as laptops and notebooks. Portable computers are rapidly developing the capability to include most of the peripheral devices and high speed circuitry previously implemented only on conventional desktop personal computers. As the number of peripheral devices supported by portable computers increases, so does the amount of power consumed by the computer. As a result, it has become increasingly difficult to provide users with the maximum functionality and still maintain a reasonable battery life.
Many portable laptop computers and similar systems incorporate rechargeable batteries, typically of Nickel Cadmium (NiCd) or Nickel Metal Hydride (NiMH) construction. Such batteries are preferred because of their high capacity and longevity after repeated recharges. The batteries are recharged using a battery charging circuit connected to a power source. The charging circuit must be capable of charging the batteries in a minimum amount of time for user convenience. Importantly, the charging circuit must also be designed to minimize deterioration of the batteries that can result should the batteries overcharge, overheat or experience a short circuit. Improper application of power by the charging circuit can, in addition to reducing battery longevity, result in permanent cell damage and possibly leakage of electrolyte to surrounding components.
Conventional battery charging circuits rely upon power controller integrated circuits to regulate the current flow in performing charging operations. Current is typically regulated using a simple pulse width modulator (PWM) regulator. A comparator monitors the battery voltage and terminates the charging cycle when the voltage reaches a threshold or begins to fall significantly, this condition being known to indicate the battery is fully charged. While widely used, a variety of problems are associated with charging circuits of this type. For example, the abrupt application of charging current to the battery at the beginning of the charging cycle can result in sharp fluctuations in voltage as the battery chemistry adapts to the current input, resulting in possible damage or failure to achieve a full charge. This is likely to occur with batteries that have been in long term storage. Overcharging can also occur with conventional charging circuits. For certain batteries, the charging cycle must be terminated with the detection of a relatively small voltage decrease to avoid overcharging, yet circuit noise can easily exceed the triggering voltage of the comparator. Further, conventional chargers typically are not equipped to terminate the charging cycle or prevent charging when deleterious conditions, such as a battery short circuit, freezing, excessive temperatures or the like, are present.
What is needed is an improved battery charging circuit that controls the rate and duration of battery charging to provide an effective yet safe charge. Specifically, a desirable arrangement would include logic for gradually stepping up the rate of charge at the beginning of the charge cycle and for terminating the charge cycle when the battery is determined to be fully charged or in the event that adverse conditions occur during charging.